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  • 1
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    The impact of environment and mergers on the H I content of galaxies in hydrodynamic simulations (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-16
    Description: The instantaneous H i content of galaxies is thought to be governed by recent accretion and environment. We examine these effects within a cosmological hydrodynamic simulation that includes a heuristic galactic outflow model that reproduces basic observed trends of H i in galaxies. We show that this model reproduces the observed H i mass function in bins of stellar mass, as well as the H i richness ( $M_{\rm H\,\small {I}}$ / M * ) versus local galaxy density. For satellite galaxies in massive ( 10 12 M ) haloes, the H i richness distribution is bimodal and the median drops towards the largest halo masses. The depletion time-scale of H i entering a massive halo is more rapid, in contrast to the specific star formation rate which shows little variation in the attenuation rate versus halo mass. This suggests that, up to the halo mass scales probed here ( 10 14 M ), star formation is mainly attenuated by starvation, but H i is additionally removed by stripping once a hot gaseous halo is present. In low-mass haloes, the H i richness of satellites is independent of radius, while in very massive haloes they become gas-poor towards the centre, confirming the increasing strength of the stripping with halo mass. Mergers somewhat increase the H i richness and its scatter about the mean relation, tracking the metallicity in a way consistent with it arising from inflow fluctuations, while star formation is significantly boosted relative to H i .
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    The impact of environment and mergers on the H I content of galaxies in hydrodynamic simulations (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-27
    Description: The instantaneous H i content of galaxies is thought to be governed by recent accretion and environment. We examine these effects within a cosmological hydrodynamic simulation that includes a heuristic galactic outflow model that reproduces basic observed trends of H i in galaxies. We show that this model reproduces the observed H i mass function in bins of stellar mass, as well as the H i richness ( $M_{\rm H\,\small {I}}$ / M * ) versus local galaxy density. For satellite galaxies in massive ( 10 12 M ) haloes, the H i richness distribution is bimodal and the median drops towards the largest halo masses. The depletion time-scale of H i entering a massive halo is more rapid, in contrast to the specific star formation rate which shows little variation in the attenuation rate versus halo mass. This suggests that, up to the halo mass scales probed here ( 10 14 M ), star formation is mainly attenuated by starvation, but H i is additionally removed by stripping once a hot gaseous halo is present. In low-mass haloes, the H i richness of satellites is independent of radius, while in very massive haloes they become gas-poor towards the centre, confirming the increasing strength of the stripping with halo mass. Mergers somewhat increase the H i richness and its scatter about the mean relation, tracking the metallicity in a way consistent with it arising from inflow fluctuations, while star formation is significantly boosted relative to H i .
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Tracing inflows and outflows with absorption lines in circumgalactic gas (2014)
    Oxford University Press
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    Publication Date: 2014-08-30
    Description: We examine how H i and metal absorption lines within low-redshift galaxy haloes trace the dynamical state of circumgalactic gas, using cosmological hydrodynamic simulations that include a well-vetted heuristic model for galactic outflows. We categorize inflowing, outflowing, and ambient gas based on its history and fate as tracked in our simulation. Following our earlier work, showing that the ionization level of absorbers was a primary factor in determining the physical conditions of absorbing gas, we show here that it is also a governing factor for its dynamical state. Low-ionization metal absorbers (e.g. Mg ii ) tend to arise in gas that will fall on to galaxies within several Gyr, while high-ionization metal absorbers (e.g. O vi ) generally trace material that was deposited by outflows many Gyr ago. Inflowing gas is dominated by enriched material that was previously ejected in an outflow; hence, accretion at low redshifts is typically substantially enriched. Recycling wind material is preferentially found closer to galaxies, and is more dominant in lower mass haloes since high-mass haloes have more hot gas that is able to support itself against infall. Low-mass haloes also tend to re-eject more of their accreted material, owing to our outflow prescription that employs higher mass loading factors for lower mass galaxies. Typical H i absorbers trace unenriched ambient material that is not participating in the baryon cycle, but stronger H i absorbers arise in cool, enriched inflowing gas. Instantaneous radial velocity measures of absorbers are generally poor at distinguishing between inflowing and outflowing gas, except in the case of very recent outflows. These results suggest that probing halo gas using a range of absorbers can provide detailed information about the amount and physical conditions of material that is participating in the baryon cycle.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    Baryon cycling in the low-redshift circumgalactic medium: a comparison of simulations to the COS-Halos survey (2016)
    Oxford University Press
    Details
    Publication Date: 2016-04-30
    Description: We analyse the low-redshift ( z 0.2) circumgalactic medium (CGM) by comparing absorption-line data from the COS-Halos survey to absorption around a matched galaxy sample from two cosmological hydrodynamic simulations. The models include different prescriptions for galactic outflows, namely hybrid energy/momentum driven wind (ezw), and constant winds (cw). We compare equivalent widths, covering factors, ion ratios, and kinematics. Both wind models show generally 1 agreement with these observations for H  i and certain low-ionization metal lines, but poorer agreement with higher ionization metal lines including Si  iii and O  vi that are well observed by COS-Halos. This suggests that both models predict too much cool, metal-enriched gas and not enough hot gas, and / or that metals are not sufficiently mixed. This may reflect our model assumption of ejecting outflows as cool and unmixing gas. Our ezw simulation includes a heuristic prescription to quench massive galaxies by superheating interstellar medium gas. This produces low-ionization absorption broadly consistent with observations, but substantial O  vi absorption inconsistent with data, suggesting that gas around quenched galaxies in the real Universe does not cool. At impact parameters of 50 kpc, recycling winds dominate the absorption of low ions and even H  i , while O  vi generally arises from metals ejected 1 Gyr ago. The similarity between the wind models is surprising, since they differ substantially in the amount and phase distribution of halo gas. We show that this similarity owes mainly to our comparison at fixed stellar (not halo) mass, suggesting that CGM properties are more closely tied to galaxy's stellar (versus halo) mass.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Hydrogen and metal line absorption around low-redshift galaxies in cosmological hydrodynamic simulations (2013)
    Oxford University Press
    Details
    Publication Date: 2013-05-21
    Description: We study the physical conditions of the circumgalactic medium (CGM) around z  = 0.25 galaxies as traced by H i and metal line absorption, using cosmological hydrodynamic simulations that include galactic outflows. Using lines of sight targeted at impact parameters from 10 kpc to 1 Mpc around galaxies with halo masses from 10 11 –10 13 M , we study the physical conditions and their variation with impact parameter b and line-of-sight velocity v in the CGM as traced by H i , Mg ii , Si iv , C iv , O vi and Ne viii absorbers. All ions show a strong excess of absorption near galaxies compared to random lines of sight. The excess continues beyond 1 Mpc, reflecting the correlation of metal absorption with large-scale structure. Absorption is particularly enhanced within about v  〈 300 km s –1 and roughly 300 kpc of galaxies (with distances somewhat larger for the highest ion), approximately delineating the CGM; this range contains the majority of global metal absorption. Low ions like Mg ii and Si iv predominantly arise in denser gas closer to galaxies and drop more rapidly with b , while high ions O vi and Ne viii trace more diffusely distributed gas with a comparatively flat radial profile; C iv is intermediate. All ions predominantly trace T ~ 10 4-4.5 K photoionized gas at all b , but when hot CGM gas is present (mostly in larger haloes), we see strong collisionally ionized O vi and Ne viii at b ≤ 100 kpc. Larger halo masses generally produce more absorption, though overall the trends are not as strong as that with impact parameter. These findings arise using our favoured outflow scalings as expected for momentum-driven winds; with no winds, the CGM gas remains mostly unenriched, while our outflow model with a constant velocity and mass loading factor produce hotter, more widely dispersed metals.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Early spectra of the gravitational wave source GW170817: Evolution of a neutron star merger (2017)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2017-12-22
    Description: On 17 August 2017, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after the merger. Over the first hour of observations, the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measured the photosphere cooling from 11,000–900+3400 to 9300–300+300 kelvin, and determined a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a began displaying broad features after 1.46 days and evolved qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process–enriched neutron star ejecta, whereas the blue component requires high-velocity, lanthanide-free material.
    Keywords: Astronomy
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 7
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    Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis (2017)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2017-12-22
    Description: On 17 August 2017, gravitational waves (GWs) were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB 170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical, and infrared light curves of SSS17a extending from 10.9 hours to 18 days postmerger. We constrain the radioactively powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in rapid neutron capture (r-process) nucleosynthesis in the universe.
    Keywords: Astronomy
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 8
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    GW150914: spin-based constraints on the merger time of the progenitor system (2016)
    Oxford University Press
    Details
    Publication Date: 2016-08-08
    Description: We explore the implications of the observed low spin of GW150914 within the context of stellar astrophysics and progenitor models. We conclude that many of the recently proposed scenarios are in marked tension with this observation. We derive a simple model for the observed spin in the case that the progenitor system was a field binary composed of a black hole (BH) and a Wolf–Rayet star and explore the implications of the observed spin for this model. The spin observation allows us to place a lower limit for the delay time between the formation of the BH+BH binary and the actual merger, t merge . We use typical values for these systems to derive t merge 10 8 yr, which proves to be an important diagnostic for different progenitor models. We anticipate the next series of events, and the associated spin parameters, will ultimately yield critical constraints on formation scenarios and on stellar parameters describing the late-stage evolution of massive stars.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    The neutral hydrogen content of galaxies in cosmological hydrodynamic simulations (2013)
    Oxford University Press
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    Publication Date: 2013-08-22
    Description: We examine the global H i properties of galaxies in quarter billion particle cosmological simulations using gadget -2, focusing on how galactic outflows impact H i content. We consider four outflow models, including a new one (ezw) motivated by recent interstellar medium simulations in which the wind speed and mass loading factor scale as expected for momentum-driven outflows for larger galaxies and energy-driven outflows for dwarfs ( 〈 75 km s –1 ). To obtain predicted H i masses, we employ a simple but effective local correction for particle self-shielding and an observationally constrained transition from neutral to molecular hydrogen. Our ezw simulation produces an H i mass function whose faint-end slope of –1.3 agrees well with observations from the Arecibo Fast Legacy ALFA survey; other models agree less well. Satellite galaxies have a bimodal distribution in H i fraction versus halo mass, with smaller satellites and/or those in larger haloes more often being H i deficient. At a given stellar mass, H i content correlates with the star formation rate and inversely correlates with metallicity, as expected if driven by stochasticity in the accretion rate. To higher redshifts, massive H i galaxies disappear and the mass function steepens. The global cosmic H i density conspires to remain fairly constant from z  ~ 5 -〉 0, but the relative contribution from smaller galaxies increases with redshift.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Nebular Hα emission in Type Ia supernova 2016jae (2021)
    EDP Sciences
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    Publication Date: 2021-08-01
    Description: There is a wide consensus that Type Ia supernovae (SNe Ia) originate from the thermonuclear explosion of CO white dwarfs (WDs), with the lack of hydrogen in the observed spectra as a distinctive feature. Here, we present supernova (SN) 2016jae, which was classified as an SN Ia from a spectrum obtained soon after its discovery. The SN reached a B-band peak of −17.93 ± 0.34 mag, followed by a fast luminosity decline with sBV0.56 ± 0.06 and inferred Δm15(B) of 1.88 ± 0.10 mag. Overall, the SN appears to be a ‘transitional’ event between a ‘normal’ SN Ia and a very dim SN Ia, such as 91bg-like SNe. Its peculiarity is that two late-time spectra, taken at +84 and +142 days after the peak, show a narrow line of Hα (with full width at half maximum of ∼650 and 1000 km s−1, respectively). This is the third low-luminosity and fast-declining SN Ia, after SN2018cqj/ATLAS18qtd and SN2018fhw/ASASSN-18tb, found in the 100IAS survey to show a resolved narrow Hα line in emission in its nebular-phase spectra. We argue that the nebular Hα emission originates in an expanding hydrogen-rich shell (with velocity ≤1000 km s−1). The hydrogen shell velocity is too high to be produced during a common envelope phase, though it may be consistent with some material stripped from an H-rich companion star in a single-degenerate progenitor system. However, the derived mass of this stripped hydrogen is ∼0.002–0.003 M⊙, which is much less than that expected (〉 0.1 M⊙) from standard models for these scenarios. Another plausible sequence of events is a weak SN ejecta interaction with an H shell ejected by optically thick winds or a nova-like eruption on the CO WD progenitor some years before the SN explosion.
    Print ISSN: 0004-6361
    Electronic ISSN: 1432-0746
    Topics: Physics
    Published by EDP Sciences
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